python 源码常见的日志用法io.StringIO()和sys.stdout

经常看到源码里面有相关的日志操作特地整理一下

1. 基础知识

from io import StringIO

#StringIO模块主要用于在内存缓冲区中读写数据
s = StringIO()
s.write("www.baidu.com
")
s.write("www.google.com")
print(s.getvalue())
'''
www.baidu.com
www.google.com
'''

2. 具体用法

'''
test1.py
'''
# redirect sys.stdout to a buffer
import sys, io
stdout = sys.stdout   #用于恢复sys.stdout
sys.stdout = io.StringIO()

# call module that calls print()
import test2 
test2.test()   #调用test2的print

# get output and restore sys.stdout
output = sys.stdout.getvalue()
sys.stdout = stdout  #恢复sys.stdout

print(output)
'''
in  test2
'''

'''
test2.py
'''
def test():
    print('in ',__name__)

3. 源码中的使用

class OutputRedirector(object):
    """ Wrapper to redirect stdout or stderr """
    def __init__(self, fp):
        self.fp = fp

    def write(self, s):
        self.fp.write(s)

    def writelines(self, lines):
        self.fp.writelines(lines)

    def flush(self):
        self.fp.flush()

stdout_redirector = OutputRedirector(sys.stdout)
stderr_redirector = OutputRedirector(sys.stderr)

...
TestResult = unittest.TestResult

class _TestResult(TestResult):
...

    def startTest(self, test):
        TestResult.startTest(self, test)
        # just one buffer for both stdout and stderr
        self.outputBuffer = io.StringIO()
        stdout_redirector.fp = self.outputBuffer
        stderr_redirector.fp = self.outputBuffer
        self.stdout0 = sys.stdout
        self.stderr0 = sys.stderr
        sys.stdout = stdout_redirector
        sys.stderr = stderr_redirector